Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core, although in an alternative configuration the fan may be driven via a gear chain from a turbine.
The fan comprises an array of radially extending fan blades mounted on a rotor and will usually provide upwards of seventy-five percent of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core through a bypass duct.
The fan is surrounded by a fan containment system and is typically located at the front end of the gas turbine engine. Located in an upstream position, the fan and fan containment system must be designed to withstand the rigours of normal operation, for instance ice and other foreign object ingestion. Additionally, the fan and fan containment system must withstand, as far as possible, a bird striking the engine. In the extreme case, upon ingestion of a bird or other foreign object for example, a fan blade may be released from the rotor. In such an event the fan containment system must be designed to contain the released fan blade so as to minimise damage to the aircraft and its vital operating systems. The fan containment system therefore has a dual purpose of forming a portion of the outer duct of the fan system and retaining detached fan blades in the event of catastrophic failure.
The major component of the fan containment system is a fan case. It is known to construct the fan case as a generally cylindrical or frustoconical containment ring surrounding the fan. The fan case may not be strictly cylindrical and may follow, from front to back, the profile of the tips of the fan blade in the axial direction of the fan. It is known to provide relatively thick metal containment rings to ensure containment of a released fan blade. It is also known to provide locally thickened isogrid metallic containment rings. Furthermore, it is also known to wrap a fibrous material such as Kevlar around a relatively thin metallic containment ring. In the event that a fan blade is released from the rotor and passes through the relatively thin metallic containment ring, the fibrous material contains the fan blade. Conventionally, it is necessary to provide a fan track liner between the fan and the fan case.
A conventional fan containment arrangement 100 is illustrated in FIG. 2 and surrounds a fan 12 comprising an array of radially extending fan blades 40. Each fan blade 40 has a leading edge 44 and fan blade tip 42. The fan containment arrangement 100 comprises a fan case 150. The fan case 150 has a generally frustoconical or cylindrical containment portion 162 and a hook 160. The hook 160 is positioned axially forward of an array of radially extending fan blades 40. A fan track liner 152 is mechanically fixed or directly bonded to the fan case 150. The fan track liner 152 may be adhesively bonded to the fan case 150. The fan track liner 152 is provided as a structural filler to bridge a deliberate gap provided between the fan case 150 and the fan 12. The gap allows for a strong containment fan case whilst compensating for normal movements the fan rotational envelope. Such out-of-round or wobble movements may be caused by out-of-balance or gyroscopic effects.
The fan track liner 152 has, in circumferential layers, an attrition liner 154, a honeycomb layer 158 and a septum 156. The septum 156 acts as a bonding layer between the attrition liner 154 and the honeycomb layer 158. The honeycomb layer 158 may be an aluminium honeycomb. The tips 42 of the fan blades 40 are intended to pass as close as possible to the fan track liner 152 when rotating. The attrition liner 154 is therefore designed to be abraded away by the fan 12 during initial operation and normal operational movements of the fan 12 to ensure the gap between the rotating fan blade tips 42 and the fan track liner 152 is as small as possible. The fan blades 40 effectively make their own ‘track’ or path in the attrition liner 154 when the engine 10 is first run up. During normal operations of the gas turbine engine, ordinary and expected movements of the fan blade 40 rotational envelope cause abrasion of the attrition liner 154. This allows the best possible seal between the fan 12 and the fan track liner 152 and so improves the effectiveness of the fan 12 in driving air through the engine.
The purpose of the hook 160 is to ensure that, in the event that a fan blade 40 detaches from the rotor, the fan blade 40 will not be ejected through the front, or intake, of the gas turbine engine. During such a fan-blade-off event, the fan blade 40 travels rapidly outwards as a centripetal force no longer maintains the fan blade's rotational trajectory. Impact with the cylindrical containment portion 162 of the fan case 150 prevents the fan blade 40 from travelling any further in a radially outward direction. The fan blade 40 will also move forwards in an axial direction as the blade 40, although no longer retained in position on the rotor, responds to the reaction force acting on the blade 40 from the air passing through the fan 12. This axially forward and radially outward motion results in the leading edge 44 of the fan blade 40 colliding with the hook 160 in the region of the fan blade tip 42. Thus the fan blade 40 is captured by the hook 160 and further axially forward movement is prevented. The cylindrical containment portion 162 and hook 160 of fan case 150 therefore combine to contain the released fan blade 40. Thus the fan blade 40, or fragments thereof, are less likely to cause damage to structures outside of the gas turbine engine casings, in particular the aircraft fuselage itself.
As can be seen from FIG. 2, for the hook 160 to function effectively, a released fan blade 40 must penetrate a fan track liner 152 in order for its forward trajectory to intercept with the hook. If the attrition liner 154 is too hard then the released fan blade 40 may not penetrate the fan track liner 152. The released fan blade 40 is therefore more likely to skip over the fan track liner 152, thus missing the hook 160 entirely, and exit from the front of the engine. Such an event where a fan blade exits from the front of the gas turbine engine would be classed as an uncontained failure.
However, the fan track liner 152 must also be stiff enough to withstand the rigours of normal operation without sustaining damage. This means the fan track liner 152 must be strong enough to withstand ice and other foreign object impacts without exhibiting damage for example. Thus there is a design conflict, where on one hand the fan track liner 152 must be hard enough to remain undamaged during normal operation, for example when subjected to ice impacts, and on the other hand allow the tip 42 of the fan blade 40 to penetrate the liner. It is a problem of balance in making the fan track liner 152 sufficiently hard enough to sustain foreign object impact, whilst at the same time, not be so hard as to alter the preferred hook-interception trajectory of a fan blade 40 released from the rotor.
This problem is further exacerbated by the tendency in modern gas turbine engines to provide a fan having fewer, but larger, fan blades. These larger fan blades tend to have a longer chord length. It follows then, that this larger chord length leads to a fan blade which is likely to have increased difficulty in penetrating the fan track liner in the event of a fan blade being released. This is because the fan blade tips will have larger cross-sectional area. A larger cross-sectional area means the energy contained within a released fan blade will be spread over a greater area of the fan track liner; therefore, the likelihood of the fan blade tips penetrating the fan track liner is reduced. Further still, if any gas turbine engine fan is operating at a relatively lower rotational speed, then it is more difficult to guarantee a released fan blade will penetrate the fan track liner, irrespective of the cross-sectional area of the fan blade tip.